When producing semiconductors to project structures onto a substrate coated with a photosensitive resist, it is customary to use lens systems that are assembled from a multiplicity of individual lenses. The lens project the patterns transirradiated by light of a mask into the photosensitive resist with high contrast. In order to avoid instances of contamination, these lens systems are sealed in an airtight fashion in housings and are purged, for example, with dried air of high purity or with nitrogen. These gases or gas mixtures hardly interact with the lens material, and undesired depositions on the lens surface can be avoided.
However, instances of lens degradation occur evermore frequently, after lengthy use of the lenses, for example, in exposing semiconductor wafers by projecting patterns at wavelengths of, for example, 365 nm and lower. Reasons for this may reside in internal or external sources of contamination. Internal sources of contaminating gas or gas mixtures are, for example, oils or sealing rings that are used in assembling the lens systems in the housings and gradually degas.
External sources are, for example, caused by the clean room air, which is not entirely free of contamination, in the neighborhood of the projection apparatuses, for example, wafer steppers or scanners. The tightness of the lens housings and of the feed lines can lessen with time, while the substances can also pass from the neighborhood into the sources of gas or gas mixture provided for the purging.
Due to transition of the wavelengths used for exposure from 365 nm (i line) in the past to 248 nm (KrF), 193 nm (ArF), 157 nm (F2) and even shorter wavelengths, there is also a rise in the energy throughput through the lenses. In particular, the interactions, induced by energy, between the lens surface and the surrounding gases will increase further in the future, thus requiring greater attention to be paid to depositions, and therefore to effects of turbidity and degradation of the lens. The action of degraded lenses can consist in the disappearance or the falling over of paths in the photoresist layers, or else in the occurrence of remains of photoresist residues. The effects manifesting themselves in lens aberrations are difficult to qualify, and therefore lead by the time the problems are detected to a high level of product losses (loss in yield).
The oils are burnt in, for example, by high-energy radiation as extended spots on the lens surfaces. In the case of CaF lenses for use of wavelengths at 193 nm, turbidity effects are known that lead, through photochemical reactions of the gas mixture, to the formation of sulfur- or phosphorus-containing salts on the lens surface.
To recognize these problems at an early stage, inline tests in which samples of semiconductor wafers coated with a photosensitive resist have been exposed with the aid of one or more test reticules or test masks have been used. Establishing a shift in focus, possibly as a function of the pattern sizes, has been an indication of increasing degradation of the lens based on instances of contamination in the gases or gas mixtures being fed for purging the lenses. These tests require valuable equipment time and permit errors to be detected only in retrospect.
Detection and prevention of lens degradations at an early stage is desirable. Also, an increase in the production yield of semiconductors, and improved the productivity of a projection apparatus for exposing substrates are desirable.